When insulating high electrical voltages, the avoidance of partial discharges is always a necessity. Such discharges, resulting for example from local increases in the electric field, lead to instances of damage in the protective layer, in particular in the case of composite insulators, as a result of which the service life is reduced. In the case of composite insulators, measures to avoid local increases in the electric field are accordingly of great significance. Known for example as a suitable measure for high-voltage insulators are shielding electrodes, which are attached to the voltage-carrying fittings and help to avoid increases in the electric field there, at the ends of the fittings.
A great problem of high-voltage insulators in this respect is the extremely inhomogeneous distribution of the variation in voltage along their length. A reason for this is stray capacitances of the insulator to ground. A further problem is local discharges on soiled insulators, produced for example by increases in the electric field where there has been local drying.
To avoid local increases in the electric field, WO 2009/100904 A1 discloses providing at least certain portions of a composite insulator with a field control layer, which comprises field-influencing particles. Such particles have, for example, a resistive or capacitive effect or are semiconducting, and, as a result of a non-linear relationship between a corresponding electrical variable and the voltage, contribute to reducing sudden changes in voltage along the insulator. Mentioned in particular are microvaristors of ZnO, which above a threshold voltage display an abrupt reduction in the electrical resistance.